U.S. Pat. No. 4,693,990 to Hiroyuki et al. discloses an olefin polymerization catalyst that includes a phthalate electron donor component. This catalyst is produced by contacting a high dihydrocarbylmagnesium compound, a halogen-containing alcohol, diisobutyl-phthalate and titanium tetrachloride. It is emphasized that the electron donor utilized in the formation of this catalyst is distinguished from the class of electron donors generically defined as a heterocyclic fused ring compound substituted with at least one oxygen atom.